Optical fiber is quickly replacing copper wire and coaxial cable for long-distance transmission in the telephone network. It is also being extended closer to the end user in the local telephone and cable networks. Typically, it has simply replaced the copper line in a point-to-point architecture in which an electrical signal modulates a laser transmitter that produces the single-frequency light conveyed on the fiber. At the receiving end, an optical detector demodulates the optical signal to an electrical signal equivalent to that carried on the copper line. Switching at network nodes is performed on the electrical signals. Therefore, each node requires demodulation and modulation, which at the desired high data rates demand expensive components. High-speed networks almost always imply that the physical channel is time division multiplexed (TDM) between a number of lower-speed logical channels, and the switch allows the different logical channels to be switched in different directions.
Such an approach is, however, limited in the amount of data that can be carried by a single fiber. The electronics at the transmitter and receiver have been pushed to 2.5 gigabits per second (2.5.times.10.sup.9 Gb/s), and further increases become difficult. About the same limitation in data pulse rate is presented by optical dispersion in the fiber over reasonable transmission lengths. Such data rates utilize only a tiny portion of the bandwidth available in optical fiber, which is at least 30 terahertz (3.times.10.sup.13 Hz).
The aggregate data rate can be significantly increased by wavelength division multiplexing (WDM) in which an array of lasers are tuned to output different optical frequencies (wavelengths) and are modulated by different electrical data signals. Their multi-wavelength outputs are combined onto a single fiber, and at the receiving end optical or other means separate the optical-frequency components to be detected by different optical detectors. Although WDM affords large increases in data throughput proportional to the number of wavelengths, it does not, at least as described above, address the problem of demodulating and modulating at each switching node and the requirement for expensive high-capacity switches. These problems are described by Zah et al. in "Multiwavelength light source with integrated DFB laser array and star coupler for WDM lightwave communications," International Journal of High Speed Electronics and Systems, vol. 5, no. 1, 1994, pp. 91-109 (hereinafter Zah I), and in "Monolithic integrated multiwavelength laser arrays for WDM lightwave systems," Optoelectronics-Devices and Technologies, vol. 9, no. 2, 1994 (hereinafter Zah II), and further references are cited therein.
New network architectures, generally referred to as all-optical networks, are being developed to solve these problems. Green, Jr. gives a general discussion in Fiber Optics Networks, (Prentice-Hall, 1993), especially at pp. 353-391. All optical networks are expected to switch optical signals without the necessity of converting them down to electrical signals. Brackett et al. describe one such architecture in "A scalable multiwavelength multihop optical network: A proposal for research on all-optical networks," Journal of Lightwave Technology, vol. 11, 1993, pp. 736-753. A very simplified version of such a WDM all-optical network, shown in FIG. 1, relies on four optical wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4 for connecting four end stations E.sub.1, E.sub.2, E.sub.3, E.sub.4. A receiving optical fiber 10 and a transmitting optical fiber 12 connect each end station to a coupler 14. Each end station receives signals at one wavelength from the receiving optical fiber 10 but can transmit on the transmitting optical fiber 12 at one or all of the other wavelengths associated with respective ones of the other end stations. For example, end station E.sub.1 receives data on an optical carrier having wavelength .lambda..sub.1 and transmits data bound for end station E.sub.2 on an optical carrier having wavelength .lambda..sub.2, etc. The coupler 14 receives a signal from a transmitting optical fiber 12 at a given wavelength from one end station and retransmits it on the receiving optical fiber 10 to the end station with which that wavelength is associated. In a rudimentary form, the coupler is a star coupler that transmits all signals received from transmitting optical fibers 12 to all receiving optical fibers 10 while the end stations process only the wavelength signals assigned to them. However, the coupler 14 may be considerably more complex than a star coupler. In general, it may be a communication network with nodes and end stations therein, but the general concepts of FIG. 1 nonetheless apply. All-optical networks of more complexity are described by Green, Jr. at pp. 353-391.
Although the optical switching units of the optical coupling unit present some of the most innovative challenges of the components in the all-optical network, the laser transmitters may be the limiting technology. Multiwavelength laser arrays are well known that are integrated on a single optoelectronic chip and that generate laser light at a number of fairly well defined wavelengths. The light propagates parallel to the plane of the chip and generally exits the chip at its edge. These laser arrays have an array of distributed feedback (DFB) semiconductor lasers in which the free-space emission wavelength .lambda. of the individual distributed feedback lasers is determined by the period .LAMBDA. of a grating longitudinally imposed on the separate lasers and n.sub.eff, the effective refractive index of the waveguide. The relationship is simply given by EQU .lambda.=2n.sub.eff .LAMBDA. (1)
The general theory and structure of DFB and equivalent lasers is described by Green, Jr. ibid., especially at pp, 200-204. The array is typically designed to have a number N of lasers whose output wavelengths are separated by .DELTA..lambda. so that the array is represented by a lasing wavelength comb. In extended networks, erbium-doped fiber amplifiers are used to maintain signal levels. These amplifiers have a relatively narrow amplification band. Also, optical switches and lasers have limited spectral resolution. In view of the restraints imposed by available components, the presently envisioned architecture for an all-optical network is limited to a channel separation .DELTA..lambda..apprxeq.4 nm or somewhat less and the number of channels are limited to the bandwidth of erbium-doped fiber amplifiers of approximately 35 nm.
In reality, such laser arrays are very difficult to achieve because many factors affect the wavelength .lambda., which must be very tightly controlled. For such a network, each laser transmitter within the network must have its frequencies calibrated with the other laser transmitters in the network so that all the combs coincide. Active temperature control can partially solve the problem. Insofar as the comb spacing .DELTA..lambda. is maintained constant but the comb of one array is uniformly displaced up or down by fabricational variations, the comb can be brought into registry with the system standard by varying the array's temperature. The lasing wavelength of a DFB laser red shifts at a rate of about 0.1 nm/.degree.C. (12.5 GHz/.degree.C. at 1.55 .mu.m) due to the temperature dependence of the waveguide effective refractive index. But, such temperature control has the potential disadvantage of requiring the lasers to operate far from their design temperature.
Furthermore, the comb wavelength spacing .DELTA..lambda. for a laser array cannot be guaranteed and is subject to its own variation .delta..lambda. due to a number of non-uniformities. See Zah I for a full discussion. Unless the variations .delta..lambda. for all the N lasers in an array can be kept below a minimum value (which necessarily decreases as .DELTA..lambda. decreases with increasing N), the array is defective for use within the network. If the yield per laser is y.sub.1, that is, a fraction y.sub.1 of lasers have an acceptable wavelength tolerance .delta..lambda..sub.max, the yield y.sub.a for an array of N lasers is EQU y.sub.a =y.sub.1.sup.N ( 2)
Thus, yields can drop to unacceptably low levels for the larger arrays required of realistic networks, particularly if the single-laser yields y.sub.1 are marginal to begin with.